In the aerospace industry and other industries with similar problems there are increasing performance demands for structural components which are light weight, have a high strength weight ratio, and for some applications which require high specific stiffness.
For many years, one of the standard structural configurations of the aircraft industry is the use of "honeycomb", where there are two outer skins, with the honeycomb core being bonded to the internal skin surfaces. Metal such as aluminum, titanium, nickel, and their alloys have been fabricated as honeycomb. Honeycomb structure is used commonly in static structures such as panels, and have also been incorporated as aircraft engine components.
However, honeycomb has certain inherent problems. For example, there is a limit to the inherent stiffness of structures made from "thin section" stitched-foil honeycomb interlayers. Such structures are expensive to fabricate from raw material to the finished part. Further, the internal configuration of honeycomb-reinforced components is difficult to inspect nondestructively with any precision. Another problem is that honeycomb structure is difficult (and as a practical matter impossible in some instances) to repair effectively.
Conventional honeycomb has two bonded areas at the surface of the skins, and these are located in highly stressed zones when there is flexural loading on the honeycomb panel. Further, the joint strength is inherently limited by the small cross-sectional area of the edges of the core members. Failure at even a small portion of the bond area can result in failure of the panel to meet its functional requirements. Also, honeycomb core properties are limited, especially in shear capabilities, due to the orientation of the honeycomb structure, the depth of the core, and the fillet width sizes of the bonding region.
Although the benefits of honeycomb structures have long been recognized, because of the difficulties or the problems such as those expressed above, there have been attempts in the prior art to form other panel structures or the like with a high strength to weight ratio. For example, in three U.S. Pat. Nos. (U.S. Pat. No. 4,113,549 issued Sept. 12, 1978, U.S. Pat. No. 4,137,118 issued Jan. 30, 1979 and U.S. Pat. No. 4,725,334 issued Feb. 16, 1988) there are disclosed panel structures having integral reinforcing ribs, where the ribs are configured with undercuts to form an T-section so as to optimize efficiency in terms of section modulus and to produce a high strength to weight ratio. Because of the ease of fabrication, superior performance and other attributes, such structures can advantageously be substituted for structures that are honeycomb or skin-and-stringer reinforced.